Lithiated cobalt dioxide and lithiated nickel dioxide are compounds having a so-called .alpha.-NaFeO.sub.2 type structure, in which lithium ions are regularly arranged in a layered manner alternately with cobalt ions or nickel ions, which is arranged vertically to the closest packing layer of oxygen ions. Due to this structure, the lithium ions in a layer can diffuse comparatively easily. Hence, lithium ions can be electrochemically doped or undoped in such a compound. Various studies have been made to use these compounds as a cathode in a lithium secondary battery. Such a lithium secondary battery is expected to play an important role as a high-performance compact secondary battery, which in the future may function as a power supply for electric vehicles or as a power storage device for load leveling.
Lithiated cobalt dioxide is already used as a cathode in lithium secondary batteries that supply power to some portable telephones and video cameras. However, lithiated cobalt dioxide, which is produced from an expensive cobalt compound, is inferior to lithiated nickel dioxide, which can be produced from an inexpensive and abundant nickel compound.
However, it is difficult to synthesize lithiated nickel dioxide having a large discharge capacity, compared with lithiated cobalt dioxide, because the charging/discharging characteristic of lithiated nickel dioxide depend largely upon the synthesis method thereof. Specifically, the difficulty is that in lithiated nickel dioxide, nickel is easily substituted at lithium sites, and therefore, the resultant compound can contain substituted nickel unless the synthesis condition is appropriate. Nickel at lithium sites inhibits the diffusion of lithium ions, thereby adversely affecting the charging/discharging characteristic of the resultant lithiated nickel dioxide.
Recently, various attempts to synthesize lithiated nickel dioxide having a large discharge capacity have been made by optimizing the synthesis condition. For example, Yamada et al (The 34th Battery Symposium, Lecture No. 2A06 (1993)) reported that lithiated nickel dioxide with an Ni oxidation number of approximately 3.0 was obtained by firing a mixture of LiOH.cndot.H.sub.2 and Ni(OH).sub.2 in oxygen at a temperature of 700.degree. C.; and then a mixture of the resultant lithiated nickel dioxide, acetylene black and polytetrafluoroethylene (hereinafter referred to as PTFE) was adhered with pressure to a current collector to manufacture a cathode, in which the discharge capacity was found to be 200 mAh/g through evaluation by a constant capacity charge. They also reported that when the charge/discharge was continued to attain the discharge capacity of 200 mAh/g, the cycle characteristic was extremely poor and the lifetime was approximately ten cycles. It was also reported to be necessary to minimize the charge capacity for the constant capacity charge in order to attain excellent cycle characteristics. It was further reported that a charge capacity of 130 mAh/g or less led to a lifetime of 100 cycles or more.
Thus, lithiated nickel dioxide has poor characteristics when it is charged/discharged at a high capacity.
In addition, lithiated nickel dioxide is inferior to lithiated cobalt dioxide in energy density when used at the same capacity. This is because lithiated nickel dioxide has a lower discharging voltage, which is defined as the characteristic of the material to be used. Generally, it is effective to increase the discharging voltage, as well as the discharge capacity, in order to obtain a secondary battery with a higher energy density. However, it has been impossible to increase the discharging voltage of pure lithiated nickel dioxide.